Various types of arrangements for inhibiting or damping the rolling motion of a floating vessel have heretofore been proposed. Such arrangements can be generally categorized as active and passive systems. Active systems include such things as gyro stabilizers, servo controlled fins, and various types of ballast transferring mechanisms which forcibly reposition ballast weight to counteract the rolling motion of the vessel.
Passive systems are systems which utilize the motion of the boat itself to move a mass in such a way as to set up counteracting forces. In known systems of the passive type, a mass is caused to oscillate back and forth under the action of gravity as the ship tips one way and then the other during the rolling action. The excitation force driving the mass back and forth laterally of the ship is the lateral component of the force of gravity produced by the roll angle of the boat. Thus when the boat tips in one direction, the weight is pulled by gravity from the high side to the low side of the vessel and then when the vessel tips in the other direction the weight is pulled back by gravity. Some combination of springs for tuning the natural frequency of the oscillating system near resonance with the roll frequency of the ship together with some damping arrangement to maintain the correct phasing between the movement of the weight and the movement of the ship is required to achieve a correcting action.
Such a system is described, for example, in prior art U.S. Pat. Nos. 3,422,782 and 3,557,735. In such known systems, the weight is part of a tuned oscillating system, that is, the system is designed such that the weight is self-centering, either by springs or by inclining of the guides for the weight toward the center. Thus the weight is driven into oscillation by the rolling action of the vessel against the restoring force of the centering means. The frequency of this oscillation is tuned to the natural roll frequency of the vessel and damping is provided to control the relative phase between the motion of the weight and the motion of the vessel. While limits on the amplitude are imposed by the structure, normal operation is for the damped weight to oscillate within these limits. The limiting structure is not designed to dissipate the kinetic energy of the moving mass but merely acts to redirect the weight back toward the center of the guide structure.
Any mechanical oscillating system operating at a condition of constant amplitude requires that the energy used to drive the system must equal the energy being absorbed by damping factors in the system. This means that the maximum damping possible is proportional to the integral of the driving or excitation force maintaining the oscillation multiplied by the amplitude or distance through which the mass is moved. In conventional mechanical systems for damping rolling of a ship, the driving force is equal to the force of gravity times the sine of the roll angle, which means that the driving force available is substantially less than the force of gravity. Since the driving force is limited, the only way that substantial damping of the system can be achieved is by using a very large value of mass or by tuning the system to very near resonance to achieve a larger distance of travel for the mass, or by using a combination of more mass plus close tuning to resonance.
To increase the mass of the moving system has the obvious disadvantage that it increases the dead weight of the vessel and takes up added space. Trying to tune the system to match the roll frequency of the boat so as to drive the oscillating system near the resonance point also has a number of disadvantages. Must hulls do not have a linear relationship between the righting movement versus the roll angle, so that the rolling frequency changes with the degree of roll. An even more serious problem is that changing the load or the load distribution of the vessel can substantially change the roll frequency. This means that the natural frequency of the oscillating system must be retuned to keep the resonance frequency near the roll frequency.